1. Field of the Invention
This invention relates to a method for measuring the tissue factor in a specimen in high sensitivity and a kit used for measurement thereof. More specifically, this invention relates to a method for immunologically measuring the human tissue factor in a human specimen in high sensitivity by a sandwich method, and a kit used for measurement thereof.
2. Description of Related Art
A tissue factor is a lipoprotein which is involved in the blood coagulation reaction and has hitherto been called "tissue thromboplastin" or "coagulation factor III", and it is a kind of conjugated protein comprising a lipid part and the protein part (apoprotein).
Because of its extremely low content in living bodies and difficulty in purification, research into of the tissue factor on a molecular level has been delayed, but the cDNA of the apoprotein of the human tissue factor was cloned and the base sequence and amino acid sequence thereof were determined, in succession, in 1987 by three research groups (Spicel, E. R., et al. : Proc. Natl. Acad. Sci. USA 84, 5148 (1987), Morrissey, J. H., et al. : Cell 84, 129 (1987), Scarpati, E. M., et al., : Biochemistry 26, 5234 (1987). They report that the apoprotein of the human tissue factor comprises 263 amino acid residues and its molecular weight by the SDS-electrophoresis method is 47 KD.
Tissue factors exist in almost all the biotissues of various animals including human beings, and particularly, are distributed, in comparatively large quantities, in the circulatory system, for example in the brain, lungs and placenta, throughout which blood vessels are provided, and vascular endothelia, and usually, are expressed and exist as a membrane protein on the membrane of the cells of these tissues.
Further, their physiological action is to bear the initiation reaction of a series of extrinsic coagulation reactions caused by damages and breakdown of blood vessels. Specifically, in the presence of Ca.sup.++, they form a molecule complex with coagulation factor VII to activate both of coagulation factors X and IX, and as a result form fibrin gel in cooperation with intrinsic coagulation reaction. Namely, tissue factors as well as factor VII are proteins playing a central role in the initiation mechanism of extrinsic coagulation reaction. (Bach, R., et al. : Biochemistry 22, 4007 (1986), Fair, D. S., et al. : J. Boil. Chem. 262, 11692 (1987), Weiss, H. I., et al.,: Blood 73, 968 (1989)).
Tissue factors having the physiological actions described above have drawn attention as an important factor for blood coagulation. Thus, they play an extremely important role in researche in the basic medicine and clinical medicine of the circulatory system to measure easily and accurately a very small quantity of the human tissue factor in a specimen.
As methods to measure the human tissue factor in a specimen, several methods have hitherto been proposed. A representative example thereof is a method, called the method of Nemerson, to measure tissue factor activity by two stage steps (Nemerson, Y., J. Clin. Invest. 47 72 (1968). To put it briefly, the measurement method is one to judge tissue factor activity by adding to a specimen a mixed solution of coagulation factors VII and X to carry out reaction for a certain time, and then adding this reaction solution to normal plasma to which a crude phospholipid is added and measuring coagulation time. However, this method has a problem that since the stability of the reagents used is low and it is difficult to measure coagulation time with good accuracy, it is difficult to measure easily a very small quantity of the human tissue factor in a solution. Further, the above method is an indirect measurement method wherein tissue factor activity is judged by measuring coagulation factor Xa, and therefore, as a matter of course, there is a limit to its accuracy. Other tissue factor measurement methods proposed based on activity measurement have each the above problems.
On the other hand, the following are reported as immunological measurement methods for tissue factors. One is radioimmunoassay (abbreviated as RIA) by Bjorklid, E., et al. (Bjorklid, E., et al. Br. J. Haematol. 39, 445 (1978)). Another is RIA by Andoh, K., et al. (Andoh, K., et al., Thromb. Res. 43, 275 (1986), Kubota, T., et al., Thromb. Haemostas. 54, 258 (1985)). Both methods are competition methods using polyclonal antibodies alone, and had a problem in specificity for the human tissue factor. Further, both methods take very long measurement time, and that is, the former takes 36 hours and the latter 51 hours. Therefore, both methods have only low practicability in view of accuracy, easiness of measurement, etc.
Thereafter, measurement methods using monoclonal antibodies were disclosed in (i) Japanese Tokuhyohei No. 503438/1989, (ii) Japanese Laid-Open Patent Publication No. 203795/1990 and (iii) Japanese Laid-Open Patent Publication No. 216054/1990. According to the researche by the present inventors, it was found that since the human tissue factor in a specimen usually forms an apoprotein-lipid complex, an immune reaction with the apoprotein is hindered, and its content in normal human blood, e.g. serum or plasma is very small and on the patents in order of Pg (pico-gram)/ml. When the methods of the (i) to (iii) above are checked from such viewpoints, specifically in the above patent (i), although in detection of the human tissue factor in a sample from a body by a sandwich method, use of TBS/TRITON (Tris buffered saline/TRITON) is disclosed as an immune reaction solution, there is no disclosure about measurement of the tissue factor in a human specimen, especially measurement of the tissue factor in human blood, e.g. serum or plasma.
As for the methods of the patents in the above (ii) and (iii), although it is disclosed to use as an immune reaction solution an aqueous solution containing a buffer solution having a pH of around neutrality such as a phosphate, Tris or Hepes buffer and a nonpolar solubilizer such as TRITON or TWEEN, there is no disclosure about measurement of the tissue factor in human blood, e.g. serum or plasma, as is the case with the method of the of the above (i).